the noise that is made when one thinks something is a little entertaining but not enough to make a full burst of laughter. also in another case you don't want to make your friend feel bad when he makes a stupid joke, so you make that noise to be a good friend

your friend says "wow thats big" and your other friend says "thats what she said (obviously stupid joke) you say "ight ight ight" to imitate real laugh, without giving the full effort to actually laugh